8.—THE GAS AND GAS-BAGS.
Several times in describing the production of the lime-light reference has been made to gas-bags; and now we will explain more fully what they are. Gas-bags are usually made of stout india-rubber cloth, wedge-shaped, their length being half as much again as their width, and the base usually equal to their width. A tap is provided and attached to the apex of the bag, so as to allow of the admission and expulsion of the gas. In order to press out the gas from a filled bag, two boards are used. These boards are of similar dimensions to the bag, and being hinged together along one edge, can be separated to allow of the introduction of the inflated gas-bag. A projecting ledge is fixed on the outside of the upper board, about eight inches from its end, against which the weight required to press out the gas can rest. The weights should be square-shaped, and the most convenient size is a half-hundredweight. When two bags are required (as in the case of the oxyhydrogen light), the double pressure-board ([Fig. 8]) should be employed; the two bags are then placed one above the other, and one set of weights only is required instead of two sets. The weight required to be used with a bag of eight feet capacity when employed for the oxycalcium lamp or jet should be about 56 lbs., but at least three times that amount should be placed on the bags for the production of the oxyhydrogen light.
Fig. 8.
It is not desirable to keep either oxygen or hydrogen gas in india-rubber bags for any length of time. Oxygen can be kept for a few days or a week, but any hydrogen gas that may remain in the bag after an exhibition should be expelled before the bag is refilled. When two bags are in use, each should be marked; O on the oxygen bag, and H on the hydrogen bag; and the bags should not be used interchangeably; i.e., hydrogen gas should not be put into an oxygen bag, nor oxygen into the hydrogen bag. In arranging the apparatus, place the pressure boards and bags where they will not be meddled with. Let the weights be securely placed on the pressure boards, so that they may not slide or roll off during the exhibition. Do not on any account allow any one to stand upon or press the boards with his foot as a substitute for, or in addition to, the weights. The lime cylinder should be wiped clean from dust before it is put upon the holder, and it should be exposed to the flame of the lamp or simple hydrogen flame some minutes before the oxygen is turned on, so that it may not fracture when the light is being produced. When all arrangements are complete, the hydrogen should be turned on and lighted, and oxygen should be gradually turned on until the requisite degree of light is obtained. With the oxycalcium light some exhibitors use the compressed oxygen, the gas being compressed into an iron cylinder, and thus bags, boards, and weights are not needed. This arrangement has the advantage of portability, but the cylinders have to be filled by means of a steam pump—a thing not always to be obtained. When a cylinder is used it should be placed as close to the jet as possible, and the flow of gas must be regulated by the valve of the cylinder, and not by the tap of the jet.
9.—OXYGEN AND HYDROGEN.
The two gases, oxygen and hydrogen, required for the production of the lime-light need now some separate notice, as it often occurs that those who use these gases for dissolving-view exhibitions have to prepare the one—sometimes both—for themselves.
Oxygen gas is a simple elementary body, and is most extensively diffused throughout our earth. It is one of the constituents of the atmosphere surrounding our globe, and also of water, as well as of nearly all the substances known as acids. It has never yet been met in its separate or uncombined state, and therefore it can only be obtained by decomposing some substance of which it is a constituent.
The material most convenient for obtaining oxygen gas for lime-light purposes is chlorate of potassa. Two parts of this material (by weight) is mixed with one part of powered black oxide of manganese (this is known as ‘oxygen mixture’). The oxide of manganese does not supply any of the oxygen, but its presence facilitates the decomposition of the chlorate of potassa. The apparatus used for generating the gas is shown at [Fig. 9]. It consists of a copper or iron vessel (A), forming the retort or generator, a glass bottle-shaped vessel (B), which is the purifier, the two being connected together by a flexible tube. It will be seen that there are two tubes proceeding from the top of the purifier, one of which reaches nearly to the bottom of the vessel, and the other only just enters into the top. The former, or long tube, is the tube by which the purifier is attached to the retort; the latter, or short tube, is that which connects the purifier with the bag.
